Microporous films have diversified pore diameters, pore shapes and pore numbers, and have found wide applications for their characteristics expressed by their peculiar structures. For example, they have been used as separation films for water treatment and enrichment because of their sieving effects by pore sizes; as adsorbent films for water absorption, oil absorption and deodorizing materials because of their large surface areas and pore spaces realized by microporosity; moisture-permeable, water-proof sheets because of their characteristics of permeating air or steam but not water; high-molecular-weight electrolyte membranes and humidification membranes useful for fuel cells and the like because of their multi-functions realized when they are filled with various materials in the pores; and liquid-crystal and cell materials.
Recently, the automotive industry, in particular, is positively studying introduction of pure electric vehicles (PEVs) and hybrid electric vehicles (HEVs), viewed from energy- and resources-saving due to heightened international activities for global environmental protection. As a result, fuel cells and large-size lithium-ion secondary cells have been actively developed as motor driving or auxiliary power sources. Moreover, electric double-layer capacitors are also greatly expected as HEV auxiliary power sources, because of their potential of charging and discharging large current in a very short time, and are being developed. Storage batteries such as lithium-ion secondary cells and electric double-layer capacitors are provided with a porous film, referred to as separator, containing an electrolyte solution between the positive and negative electrodes, because it prevents contact of these electrodes with each other and has a function of transmitting ions.
Lithium-ion secondary cells are required to be reliable and safe for extended periods, and their separators are required to impregnate and hold an electrolyte solution. Charging and discharging cycles are accompanied by expansion and contraction of the cell. Its separator, when compressed, discharges an electrolyte solution it holds, and depletion of the solution occurs, when the discharged solution is not returned back to the separator. This causes deterioration of cell performance, e.g., reduced cell capacity.
Large-size cells, in particular those for PEVs or HEVs, need much electrolyte solution charging time while they are being produced when their solution impregnating capability is insufficient, because of their large separator size. Moreover, they may encounter troubles which may deteriorate productivity or cell performance, e.g., deteriorated performance resulting from unevenly charged solution.
An electrode-active material may impale a separator, because its shape is not always smooth, to cause short-circuiting between the electrodes. Therefore, a separator is required to have a high piercing strength to prevent short-circuiting. Moreover, lithium-ion secondary cells or the like of high capacity and high output potentially generate excessive heat more frequently than conventional ones under abnormal conditions, e.g., short-circuiting and overcharging, because of larger energy quantity they contain. These cells, therefore, are provided with several measures to secure safety under abnormal conditions. One of the measures is a separator shut-down function. This function closes the separator pores, when cell temperature increases by some kind of causes, to prevent ion movement, terminates the cell reactions and thereby controls excessive heat generation. Porous polyethylene films have been extensively used for lithium cell separators, one of the reasons for which is their excellent shut-down functions. However, cells of higher energy, now under development, generate much larger quantities of heat under abnormal conditions, possibly increasing cell temperature to a high level in a short time or keeping them at high temperature for extended periods, because much time is needed for radiating the heat after the cells are shut down. Under these conditions, the separator may be contracted or broken to cause short-circuiting between the positive and negative electrodes to generate more heat.
Electric double-layer capacitors, on the other hand, are required to have higher capacity, and the separators therefor are required to be thinner. Cellulosic papers and glass fiber sheets, which have been widely used for separators, are difficult to be further thinner due to production difficulty and concerns about self-discharging. Microporous polyolefin films have been studied, because they can be potentially thinner and stronger than the conventional materials. However, they have not been commercialized for electric double-layer capacitors, because of their low electrolytic solution impregnating capability.
Studies have been extensively made to solve these problems, but have not always produced satisfactory results.
Patent Document 1, for example, discloses a porous film of polyolefin containing particles of inorganic compound, e.g., titanium oxide, aluminum oxide or potassium titanate, at 20 to 80% by mass, claiming that it can keep insulation capability even at high temperature by virtue of presence of the inorganic powder and has improved resistance to short-circuiting at high temperature. The method disclosed by the document, however, may give a film of insufficient shut-down function. Moreover, the microporous film produced by the method is limited in industrial productivity, high strength and thinness. In a conventional method, inorganic particles tend to agglomerate with each other while they are kneaded with a polyolefin resin and plasticizer. Therefore, it can reduce agglomerated particles to a limited extent. When a film not sufficiently free of agglomerated particles is stretched at a high magnification, the pore structure becomes coarse originating from the agglomerated particles, with the result that the film tends to be broken. Such a microporous film is not suited for industrial production and, at the same time, difficult to attain a high piercing strength. Moreover, in order to produce a thin film by stretching at a low magnification of 10 times or so, as disclosed by Patent Document 1, it is conceivably extruded from a die or the like with narrow dielip clearance. Decreasing the clearance, however, tends to cause troubles which can eventually break the film, e.g., deposition of resin, and formation of streaks and waves on the film.
Patent Document 2 discloses that a porous polyolefin film containing particles of silicon oxide, alumina or the like having an average diameter of 100 nm or less at a relatively low content of 1% by mass or more but less than 20% by mass can be stretched at a high magnification, and that it is resistant to short-circuiting at high temperature in spite of its high piercing strength and thinness. It also discusses that a microporous film will have a decreased strength and cause agglomeration of the inorganic particles when the inorganic particle content increases to 20% by mass or more. The porous film disclosed by Patent Document 2, however, may not exhibit sufficient capability of impregnating and retaining an electrolyte solution, because of its relatively low inorganic particle content. Moreover, it will have deteriorated shape retainability at high temperature of its melting point or higher, and is required to have still improved resistance to short-circuiting.
Patent Document 3 proposes a porous film containing a ultrahigh-molecular-weight polyolefin resin having a weight-average molecular weight of 500,000 or more, and particles having a diameter of 0.001 to 10 μm at 5 to 70% by mass. However, the porous film disclosed in the embodiment cannot have a high piercing strength, because of low magnification of 2 by 2 times at which it is stretched.
Patent Document 4 discloses a porous film composed of 30 to 85% by mass of polyolefin and 15 to 70% by mass of flat inorganic particles, where the polyolefin contains a ultrahigh-molecular-weight polyolefin having a weight-average molecular weight of 1,000,000 or more. Example 1 in the specification uses a ultrahigh-molecular-weight polyolefin having a weight-average molecular weight of 2,000,000 and a combination of rolling and biaxial stretching to achieve a high strength at an overall magnification of 75 times. However, the rolling presses the film by a planar press and does not hold the film in the biaxial directions, and increases film strength to a limited extent, because of orientation relaxing proceeding simultaneously. Moreover, the rolling at a high magnification is difficult to apply to continuous production, and unsuitable for industrial production. Still more, the rolling of the film containing a ultrahigh-molecular-weight polyolefin having a weight-average molecular weight of 1,000,000 may cause problems which lead to failure of complete shut-down, because of increased shut-down temperature and insufficient resistance increase. A film containing inorganic particles and a ultrahigh-molecular-weight polyolefin having a weight-average molecular weight of 1,000,000 or more tends to greatly increase film melt viscosity and shutdown temperature. Moreover, the film may have still increased shut-down temperature when rolled, because of progress of orientation relaxing.
Patent Document 5 discloses a porous film containing a filler of calcium carbonate, barium sulfate or the like, high-density polyethylene and low-molecular-weight compound, and stretched at a magnification of 3 times or more in each of the lengthwise and crosswise directions. It realizes a high magnification by incorporating the low-molecular-weight compound at 0.5 to 10 parts by mass per 100 parts by mass of the high-density polyethylene. However, the method disclosed by Patent Document 5 is essentially based on boundary separation to produce the porous film, and produces a film of increased porosity because of contamination with air. As a result, it has a limited capability for producing a thin film of high piercing strength.
Patent Document 1: JP-A-10-50287
Patent Document 2: JP-A-2003-292665
Patent Document 3: JP-A-2003-26847
Patent Document 4: JP-A-2000-256491
Patent Document 5: JP-A-2003-82139